As indicated above, this invention is directed to improved polymer-sheathed multi-filamentary strands that can be braided in order to cover wiring harnesses in harsh environments, such as the engine compartments of heavy trucks, construction equipment, jet aircraft, and the like. These polymer-sheathed multi-filamentary strands must avoid degradation in their physical properties despite exposure to the high temperatures and corrosive (meaning by “corrosive” to include all manner of chemical activity, including solvent action and the like) liquids found in such environments for periods of many years. Other requirements on such polymer-sheathed multi-filamentary strands will be apparent and will be discussed further below, such as abrasion resistance and the ability to be handled readily, so as to be wound onto spools and to be braided into wiring harness covers without difficulty. Cost of course is always important.
It will be appreciated that the engine compartments of vehicles such as heavy trucks, construction equipment, jet aircraft and the like are very harsh environments, and that components intended for use in such environments must meet many challenges. In particular, with engines growing to be of higher and higher power output, temperatures continue to rise in these engine compartments such that components such as polymer-sheathed multi-filamentary strands employed for braided covers for wiring harnesses face significantly more difficult temperature tolerance requirements. More specifically, polymers were previously considered satisfactory as the material for sheathing multi-filamentary strands for braided wiring harnesses covers if capable of withstanding temperatures of 280° F. (137° C.) over a period of years without stiffening, cracking, or other failure even if repeatedly flexed. The service temperatures now contemplated require these materials to withstand temperatures of 392° F. (200° C.), which is a substantial increase. At the same time, the materials must be unaffected by long-term exposure to various corrosive substances found in such environments, such as gasoline, diesel fuel, jet fuel, coolants, battery acid, oil, grease, windshield washer fluid, brake and other hydraulic fluids, steam, and the like. It is also important that the polymer sheath in particular be resistant to abrasion, as there is inevitable vibration with respect to metal components in these environments, such that if not sufficiently abrasion-resistant the sheath could chafe through, for example, at the points where the wiring harness is secured in position, leading to failure.
In addition to these service requirements, the materials of the polymer of the sheath and of the yarn must be suitably processable, meaning that their characteristics must be such that the polymer can be extruded over a multi-filamentary strand at high speed using conventional processing equipment, in order to manufacture the polymer-sheathed multi-filamentary strands, and that the polymer-sheathed multi-filamentary strands can then be braided to manufacture covers for wiring harnesses on conventional equipment, for reasons of economical production.
These requirements on the materials of the sheath and multi-filamentary strands are complex, particularly in connection with the polymer of the sheath, in that the polymer must be melted in order that it can be extruded over the multi-filamentary strand. In order that the polymer can be extruded using conventional equipment, the melting temperature of the polymer cannot be too high, yet as noted the polymer must be capable of withstanding relatively high temperatures in service over a period of many years. This places a severe constraint on the polymer. In addition, the sheathed yarn, typically of 30-40 mil diameter with about 14 mil sheath thickness must be flexible enough for winding onto cardboard tubes and downstream processing such as rewinding and braiding. If the yarn is not flexible enough, the yarn tends to slough off the cardboard tube and it becomes very difficult to handle or rewind. Additionally, if the sheath is too stiff, the yarn may spring back from the braided structure or break during the braiding operation. Similarly, the materials of the sheath and multi-filamentary strand cannot be too costly for economical production and use.
Therefore, it is an object of the invention to select materials and processing techniques that result in a polymer-sheathed multi-filamentary strand that is resistant to high temperatures, various corrosive materials, and abrasion, that is sufficiently flexible for convenient downstream processing, and wherein the materials can be processed on conventional equipment using generally conventional techniques, at reasonable cost of the ultimate product.
In a further aspect of the invention, required for use in connection with modern vehicles which are largely controlled by microprocessors, it is desired to incorporate shielding against electromagnetic interference (EMI) into the yarns making up the braided wiring harness cover, so as to prevent the wire bundle protected by the wiring harness cover from acting as a noise radiating antenna or as a receiver for outside EMI, which might distort the signals being carried, which in turn might cause various operational difficulties. Such EMI shielding is desired in many of the same applications as intended for the high-temperature resistant strands discussed above, e.g., the engine compartments of trucks and heavy equipment, jet engines, and otherwise. Provision of EMI shielding generally requires the provision of at least one grounded conductive layer or wire surrounding the wiring to be protected, which serves to absorb, drain or reflect electromagnetic energy. A polymer-coated strand suitable for being braided into a wiring harness cover incorporating EMI shielding would accordingly be a useful product.